Sahih Muslim Book 4, Hadith Number 1001.

Chapter : How the limbs should work in prostration and forbiddance to fold clothing and hair and plaiting of hair in the prayer.

This Hadith has been narrated by Ja’far b. Rabi’ with the same chain of transmitters. And in the narration transmitted by ‘Amr b. al-Harith (the words are): “When the Messenger of Allah (peace be upon him) prostrated, he spread out his arms so that the whiteness of his armpits was visible.” And in the narration transmitted by al-Laith (the words are: “When the Messenger of Allah (may peace be upon him) prostrated, he spread his hands from the armpits so that I saw their whiteness.”